We don't care how casual the wedding is and how laid-back the bride in question might be, attending in jeans and a T-shirt is not an option.
It doesn't matter if it's on the beach, in your mom's backyard, or even if you don't know the couple very well. If you're bothering to show up, the least you can do is throw on a dress or a pair of khakis and a button-up shirt -- again, assuming it's a casual ceremony. Also, just to be clear, don't even think about slipping on a pair of flip-flops. Pumps, heels, flats, loafers and sandals are a go, but plastic footwear is a huge wedding no-no. (Possible exception: If the wedding invite explicitly states that it's a "flip-flop casual" beach event, you may wear them.)